<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>布尔类型</title>
</head>

<body>
  <script>
    // 布尔类型
    let isCool = true
    console.log(isCool);

    // undefined类型
    let num
    console.log(num);  //undefined,定义了但是没给值

    // null类型

    let obj = null
    console.log(obj);  // null空的

    console.log(undefined + 1);    // NaN
    console.log(null + 1);         // 1
  </script>
</body>

</html>